1. Field of the Disclosure
The present disclosure relates to a nitride-based compound semiconductor device and a method of fabricating the same, and more particularly, to a nitride-based compound semiconductor device having improved characteristics of ohmic contact to an n-electrode and a method of fabricating the same.
2. Description of the Related Art
Laser diodes (LDs) or light-emitting diodes (LEDs) are commonly known as nitride-based semiconductor light emitting devices. LEDs are semiconductor devices that emit light of specific wavelength as electrons move from a high energy to low energy level when electricity is applied. LEDs are widely used in various applications to create light such as green light on the mother board when a hard disc spins, light on an electronic display board installed at buildings, or blinking light on a cellular phone. The LEDs have emerged as new light-emitters that provide about 1/12 the power consumption, more than 100 times the life span, and more than 1,000 times the reaction rate to electricity when compared to conventional bulbs. They are also receiving considerable attention as a promising display means such as electronic display boards because of the high brightness and low power consumption. LEDs emit light of different colors depending on the type of compound semiconductor materials used (e.g., gallium phosphide (GaP) or gallium arsenide (GaAs)). In particular, LEDs emitting red or green light have been widely used in various industrial applications as well as in home electronic appliances for over several decades.
LEDs are classified into top-emitting light-emitting diodes (TLEDs) and flip-chip LEDs (FCLEDs) depending on the direction in which light exits. In commonly used TLEDs, light exits through a p-electrode forming an ohmic contact with a p-type compound semiconductor layer. The p-electrode is typically formed of nickel (Ni)/gold (Au) on the p-type compound semiconductor layer. However, a TLED using a semi-transparent Ni/Au p-electrode has a low light utilization efficiency and low brightness. In a FCLED, light generated in an active layer is reflected by a reflective electrode and the reflected light is emitted through a substrate. The reflective electrode is made of a highly light reflective material such as silver (Ag), aluminum (Al), or rhodium (Rh). The FCLED using the reflective electrode can provide a high light utilization efficiency and high brightness.
A conventional n-electrode in a LED or LD is made of an Al—Ti based material that should be annealed at a high temperature above 600° C. In particular, the Al—Ti based material has difficulty in forming an ohmic contact on an N-polar surface of a freestanding GaN substrate. More specifically, conventionally Ti/Al or Al/Ti are deposited on the GaN substrate and then are annealed at high temperature above 600° C. to form an ohmic contact to an n-electrode. However, annealing at high temperature above 600° C. may cause thermal damage to layers formed in a stack before annealing. To prevent this problem, conventionally an n-electrode is formed on a GaN substrate before forming thereon an n-type compound semiconductor layer, an active layer, a p-type compound semiconductor layer, and a p-electrode. As described above, another drawback of an Al—Ti based material is that it can form an ohmic contact only on a Ga-polar surface. It has been known that the Al—Ti based material has difficulty in forming an ohmic contact on the N-polar surface. Thus, to overcome these problems, there is an urgent need to develop an n-electrode material that can improve the characteristics of ohmic contact to the n-electrode and improve the structure of the n-electrode.